Enhanced oil recovery processes employing a multiple-contact miscible gas mechanism have been successfully applied in a number of oil-bearing reservoirs. However, many reservoirs are confined to immiscible techniques for technical and economic reasons. For example, some reservoirs are too shallow to allow gas injection at the high pressures needed for multiple-contact miscibility. The cost of compression facilities severely limits the feasibility of high pressure gas injection for most offshore reservoirs. Compression costs are also a factor in limiting the feasibility of miscible techniques for many on-shore reservoirs. Finally, in some locations gases of optimum composition for efficient oil recovery using miscible techniques are not available. Thus, there is great incentive for technology that will increase recovery efficiency of immiscible gas processes.
Oil displacement efficiency with immiscible gas is limited by the ability of the injected gas phase to penetrate pore throats. Ability of gas to penetrate pore throats depends on gas-oil interfacial tension and the gas-oil-rock contact angle. These effects are combined and expressed in the gas-oil capillary pressure relationships.
One approach to increasing oil recovery efficiency of an immiscible gas process is to reduce gas-oil interfacial tension. Gas-oil interfacial tension drops with increasing pressure for a gas-oil pair. In fact, gas-oil interfacial tension may drop so low that miscibility between the gas and oil actually develop after multiple contacting at sufficiently high pressures. Gas-oil interfacial tension also drops as gas is enriched with gaseous components having higher oil solubility.
As such, a number of fluid injection processes have been developed to capitalize on this approach. U.S. Pat. No. 4,842,065 to McClure injects sequential slugs of a liquid surfactant and water into an oil-bearing reservoir to alter the wettability of the reservoir rock and enhance oil recovery therefrom. U.S. Pat. No. 4,846,276 to Haines injects sequential slugs of water and an immiscible gas into an oil-bearing reservoir to enhance oil recovery therefrom.
Accordingly, it is an object of the present invention to provide a sequential injection process employing immiscible fluids which enhances oil recovery from a subterranean hydrocarbon-bearing formation. It is also an object of the present invention to provide such a process which is both simple and economic to operate at an increased oil recovery efficiency. It is further an object of the present invention to provide a gas injection process for increased oil recovery which can effectively utilize a number of different gas compositions as a function of availability.